Transmission system



May 17, 1938. H NB M 2,117,698

TRANSMISSION SYSTEM Filed Jan. 11. 1936 7 FILTER FIG. 2

wva/vfoe H. 7. BUDE/VBOM Patented May 17, 1938 TRANSMISSION SYSTEM Horace '1. Budenbom,

Short Hills, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 11, 1936, Serial No. 58,727

13 Claims.

This invention relates to transmission systems and particularly to arrangements for controlling and regulating the gain in these systems.

The objects of the invention are to regulate the gain in a transmission system in such a manner as to obtain the most satisfactory transmission levels consistent with good quality; to utilize the undesirable signal components present in the system to regulate and control the transmission level; and otherwise to effect improvements in systems of this character.

Systems have been devised inthe past for utilizing the signal currents in the output circuit of an amplifier for the purpose of controlling the gain of the amplifier. And in some cases it has been suggested to make use of the noise currents, which frequently accompany the useful signal currents in a transmission system, to regulate and maintain the energy level in the system at certain desired values.

According to the present invention advantages are secured over these prior systems by means of a gain control arrangement in which the noise currents are selectively separated from the signal currents in the output circuit of an amplifier and then applied to the input control circuit of the amplifier to control the gain of the amplifier and maintain the energy level in the output circuit within certain desired limits. The separation of the noise currents is effected by providing the output transformer with aiding windings in which the useful signal energy flows to the work circuit and also with neutralizing windings in which the signal currents are balanced out but which do not fully neutralize the noise currents because of the coupling of the winding and be cause of the random nature of said currents. These unneutralized noise currents are then delivered by the neutralizing windings of the transformer to a gain control circuit, in which they are utilized to control the input potential of the amplifier.

The foregoing and other advantages of the invention will be explained more fully in the following detailed specification and will also be set forth in the appended claims.

In the drawing, which accompanies the specification and forms a part thereof, Fig. l represents any stage of translation, such as a detector or amplifier, in a wave transmission system. More specifically it shows two vacuum tubes connected in push-pull relationship for amplifying signal waves delivered to them from an incoming circuit and for delivering the amplified waves to an outgoing work circuit. And Fig. 2 is a diahad. These noise currents are a phenomenon resulting largely from thermal conditions in the physical parts of the system; they are of a random nature and occur to a large extent in accordance with laws that are not readily ascertainable. They are, however, known to be proportional to the absolute temperature of the physical elements of the system, to the resistance values of the circuit elements, and to the width of the frequency band being transmitted. Reference may be had to an article by J. B. Johnson entitled Thermal agitation of electricity in conductors published in Physical Review, volume 32, pages 97 to 109, for a discussion of the nature and magnitude of thermal noise currents. These noise currents mingle with the useful signal currents, and it is their presence that deter mines and limits the amount of amplification that canbe effected at any given stage. As the gain of the amplifier is raised, the amplified noise energy becomes a point is reached beyond which it is inadvisable to increase the gain. And it is because of this relation between the intensity of the harmful noise currents and the gain applied to the useful signals that it is rents as a measure of the permissible gain for the signal currents and to maintain the transmission level at the best values consistent with the quality required.

Referring now particularly to the drawing, there is here shown a single translating and amplifying stage in a transmission system. The amplifying stage is shown as comprising a pair of translating devices, each of which may be a space-discharge tube of any well-known type. While an amplifying stage is illustrated and described herein, it should be understood that the invention may be applied to a stage where demodulation occurs or to any other translation stage in, a transmission system. Likewise, it will be obvious that the specific construction of the amplifying or translating devices may vary.

The incoming circuit l shown in the drawing is connected to the primary winding 2 of the input transformer T. The two secondary windmore and more disturbing until possible to utilize the noise curings 3 and 4 of the transformer T are connected in series, the free terminals being connected respectively to the grid control elements of the space discharge amplifiers A and A-i. The midpoint of the connected windings 3 and t is joined by conductor 5 to a common point between the cathode elements of the amplifiers A and A-l. A condenser 6 is included in the conductor 5. The plate or anode element of the device A is connected to one terminal of the primary winding 7 of the input transformer T-i, and the anode of the other device A-! is connected to one terminal of a second primary winding 3 of said transformer. The other terminals of the windings l and 8 are connected to form a midpoint, and this mid-point is joined by a conductor 9 to the common point of the cathodes of the devices A and AI. A condenser I is in cluded in-the conductor 9. A battery I l is con nected through a choke coil IE or any other suitable impedance to the conductor 9 for the purpose of furnishing plate current to the tubes A and A-i. With the circuit arrangement thus described the amplifiers A and A--i are connected in the well-known pushpull relationship. The transformer Ti is provided with four secondary windings l3, l4, l and It. The wind ings i4 and I5 are connected to each other in series aiding and to the outgoing transmission circuit H. The other two coils l3 and I6 are connected to each other in series opposing and to the auxiliary or control circuit I8. The primary winding 7 is preferably coupled closely to windings l3 and i4 and loosely to windings l5 and i6, and the primary winding 8 in like manner is coupled closely towindings l5 and it and loosely to windings l3 and M. This coupling arrangement between the primary and secondary windings is illustrated schematically in Fig. 2, and its purpose will be discussed more fully later.

The control circuit l8, which includes the opposing windings l3 and I6, is connected to the input side of any suitable amplifier I 9. A frequency selective device 25 of any suitable kind, such as a filter, is included in the control circuit H3. The output circuit of the amplifier H! includes a resistance 2% and a rectifying device 2!. The poling of the rectifier 2! is such that the amplified current in the output circuit of the amplifier i9 always flows in the same direction, namely, the direction in which the right-hand grounded terminal 2 of the resistance 29 is positive with respect to the other terminal 23 of the resistance. The terminals of the resistance 29 are also connected across the condenser 6, and a choke coil 22 or other suitable impedance is included in the connection as shown.

The operation of the system will now be described. Assume that signal currents are flowing over the transmission line i. These currents pass through the primary Winding 2 of the transformer T and induce potentials in the secondary windings 3 and 4. These induced potentials cause in the well-known manner changes of potential on the grid elements of the tubes A and A!, resulting in turn in the flow of signal currents through the windings 1 and 8 in the output circuits of said tubes. The signal current flowing in the winding '5 bears a definite phase relation to the signal current flowing in the winding 8, and these currents cause equal potentials to be induced in those secondary windings of the transformer which have the same degree of coupling with respect to said primary windings l and 8. For example, the potential induced in the winding M from the closely coupled winding 7 is equal to the potential induced simultaneously in the winding [5 from the closely coupled winding 8. Similarly, the signal current induced in the winding 15 from the loosely coupled primary winding '5 is equal to the signal current induced in the winding M from the loosely coupled primary winding 8. These induced potentials in windings l4 and [5 add to cause a resultant signal current flow in the outgoing transmission line H. Also, since equal currents are flowing in windings I and 8, the potential induced in the winding Hi from the closely coupled winding l is equal to the potential induced in the winding is from the closely coupled winding 8, and the potential induced in the winding 53 from the loosely coupled winding 8 is equal to the potential induced in the winding it from the loosely coupled winding 1. These equal potentials in the windings I 3 and It being of opposite phase completely neutralize each other and no signal current flows in the auxiliary circuit i8. There are also present in the output circuit of the amplifier tubes, as was explained hereinbefore, disturbing currents which result from different causes and which are usually referred to as noise currents. Because of their random nature, the noise currents flowing in the primary winding 7 of the transformer are not in phase with the noise currents flowing in the other primary winding 8. Since the winding I is coupled closely to winding i 3 and loosely to winding IS, the potential induced in the winding i3 as a result of the noise current in winding 3' is greater than the opposing potential induced in the winding Hi from the winding 1.' Accordingly, there is as a result of the noise currents flowing in winding 1, a certain voltage across windings l3 and it which is obtained by subtracting from the greater potential in the winding l3 the lesser potential in the winding Hi. In a similar manner the noise currents flowing in the winding 8 at the same instant induce a greater potential in the winding l6 and a lesser potential in the winding I3, and the resultant of these two last-mentioned potentials is a voltage which is out of phase with the voltage resulting from the primary winding 'l. Therefore, there appears across the conductors of the control circuit IS a resultant voltage which causes a corresponding current to flow toward the input circuit of the amplifier !9. This current is amplified by the device 19 to produce a voltage across the resistance 29 which varies in magnitude with the intensity of the noise current flowing in circuit it. As the noise currents in the system increase in intensity, the terminal 23 of the resistance 26 becomes more and more negative with respect to the other terminal 24 of said resistance. This increases the negative potential on the grid elements of the tubes with respect to their cathodes and correspondingly reduces the gain of the amplifiers.

Since even harmonic currents, should they appear in the output circuits of the amplifiers, bear a phase relationship for the two amplifiers, operating in a push-pull manner, opposite the phase relation of the fundamentals and odd-order curcents for the two amplifiers, these even-order currents may appear in the circuit l8. That is, if the odd-order terms are in opposing phase in the common circuit 9 and coils l3 and I6 are wound to exclude these components from circuit l8, the even-order components are in phase in circuit 9 and would then appear in circuit I8.

These even-order currents may be effectively eliminated from circuit 18 by means of the filter 25. The filter 25 may be designed to exclude the first even-order component and all higher frequency components, leaving in the circuit 3 all noise currents having frequencies under that of the second harmonic. Or, to obtain a greater range of noise currents, the filter may be of the wellknown multi-peak type so designed that it eliminates the successive even-order components, allowing all noise currents having frequencies between these successive components to flow to the amplifier l9.

What is claimed is:

l. The combination in a transmission system of a transmission line having a translating device therein, input and output circuits for said device, coupling transformer in the output circuit having its windings arranged to selectively separate the thermally produced noise currents signal currents in saidoutput circuit, and .neans for utilizing said selected noise currents to control the voltage in the input circuit of said translating device.

2. The combination in a transmission system of an amplifier having input and output circuits, an incoming transmission line coupled to said input circuit, an outgoing transmission line, a coupling transformer between said output circuit and outgoing line and having its windings arranged to separate the thermally produced noise currents from the signal currents in said output circuit, a control circuit for receiving the separated noise currents from said transformer, and means responsive to the noise currents in said control circuit for applying control potentials to the input circuit of said amplifier.

3. The combination in a transmission system of an amplifier having input and output circuits, an incoming transmission line coupled to said input circuit, an outgoing transmission line, a control circuit, a transformer having a primary winding in said output circuit, a secondary wind- :or said transformer connected in said outgoing transmission line, other secondary windings for said transformer connected in said control circuit, said other secondary windings arranged to deliver to said control circuit the noise currents induced therein from said output circuit and to effectively neutralize all signal currents induced therein, and means for amplifying the noise currents in said control circuit and utilizing them to control the gain of said amplifier.

4. The combination in a transmission system of an amplifier having input and output circuits, an incoming transmission line coupled to said input circuit, an outgoing transmission line, a control circuit, a transformer having a primary winding in said output circuit, a secondary winding for said transformer for delivering to said outgoing transmission line currents induced therein from said primary winding, a pair of opposing secondary windings for said transformer connected to said control circuit, said latter windings serving to neutralize all signal currents induced therein from said primary winding and to deliver to said control circuit the unneutralized components of. the random noise currents induced iih ,rein from said primary winding, and means in said control circuit for utilizing said noise currents to control the output signal level of said amplifier.

In combination an amplifier having input and output circuits, a coupling transformer connected in said output circuit, a work circuit, a control circuit, a pair of windings for said transformer connected in aiding relation for delivering to said work circuit currents induced from said output circuit, a second pair of windings for said transformer connected in opposition for neutralizing all currents induced with equal intensities in both windings and for partially neutralizing currents induced in said windings in unequal intensities, said second pair of windings serving to deliver to said control circuit all unneutralized currents induced therein, and means for utilizing the currents in said control circuit to control the operation of said amplifier.

6. In combination an amplifier comprising two space-discharge devices, a combined output circuit for said devices having signal currents and random noise currents therein, a transformer having its primary windings connected in said output circuit, a control circuit, secondary windings for said transformer arranged to deliver to said control circuit a portion of the noise currents induced therein from said primary windings and to fully neutralize all signal currents induced therein from said primary windings, and means responsive to said noise currents in said control circuit for controlling the gain of said amplifier.

'7. The combination in a transmission system of. means for transferring energy from one part of the system to another comprising a transformer having a. plurality of secondary windings, two of which are connected in opposition, a primary winding coupled closely to one and loosely to the other of said opposing secondary windings and a second primary winding likewise coupled closely to one and loosely to the other of said opposing secondary windings, and circuit means for delivering the currents in said system to said primary windings.

8. The combination in a transmission system of an incoming circuit, two outgoing circuits, means for transferring energy from said incoming circuit to said outgoing circuits comprising a transformer having a pair of secondary windings connected in the first outgoing circuit, a second pair of secondary windings connected to the second outgoingcircuit, a primary winding closely coupled to one of the windings connected to said first outgoing circuit and to one of the windings connected to said second outgoing circuit and loosely coupled to the remaining two secondary windings, and a second primary winding closely coupled to said remaining two secondary windings and loosely coupled to the other two secondary windings.

9. The combination in a transmission system, an incoming circuit, an outgoing circuit, a coupling transformer between said circuits comprising a pair of secondary windings wound in opposing relation, a primary winding closely coupled to the first and loosely coupled to the second of said secondary windings, a second primary winding closely coupled to the second and loosely coupled tothe first of said secondary windings, an amplifier serving to deliver to said primary windings currents having both useful and harmful components, the useful currents appearing in phase in said primary windings and the harmful currents appearing out of phase in said primary windings, and means for utilizing the unneutralized currents induced in said secondary windings for controlling the operation of said amplifier.

10. The combination in a transmission system of a transmission line having a translating device-therein, input and output circuits for said device, a coupling transformer in the output circuit having its windings arranged to select thermally produced noise currents and means for utilizing said selected noise currents for controlling the voltage in said input circuit.

11. The combination in a signal transmission of a transmission line having a translating device therein, input and output circuits for said device, a transformer having a primary winding connected in said output circuit, and a control circuit for controlling the voltage in said input circuit, the primary and secondary windings of said transformer being so arranged that noise currents existing in said output circuit as a result of noise currents thermally produced in said input circuit are impressed on Said control circuit while signaling currents have no eifect on said control circuit.

12. In a transmission system, a transformer having two primary windings and a plurality of secondary windings, said primary windings being energized by currents some components of which are at all times of' substantially equal intensity in both windings and some components of which areof random intensity and occurence in the two windings, one of the secondary windings being closely coupled to one of the primary windings and loosely coupled to the other of the primary windings, and another secondary winding being closely coupled to said other primary winding and loosely coupled to said one primary winding, said secondary windings being connected in opposition.

13. In a transmission system, a transformer having two primary windings and a plurality of secondary windings, said primary windings being energized by currents, some components of which are at all times of substantially equal intensity in both windings and some components of which are of random intensity and occurrence in the two windings, one of the secondary windings being closely coupled to both of said primary windings, another secondary Winding being closely coupled to one of the primary windings and loosely coupled to the other of the primary windings, and another secondary Winding being closely coupled to said other primary winding and loosely coupled to said one primary winding, said other secondary windings being connected in opposition.

HORACE T. BUDENBOM. 

